Cary Honnold, Purdue University

As modern swine production has evolved the industry has utilized both technological and scientific advancements, as well as adapting new management systems to successfully eradicate and control many diseases. However, even with these advances, swine producers and veterinarians are still continuously being challenged by disease and poor performance. One disease that seems to have evolved with modern swine production is respiratory disease. Now, more than ever, respiratory disease and the increased costs of production incurred with it, are serving as a major source of frustration for both producers and practitioners alike. Even with the advent of new management technologies, advanced biologics, and a better understanding of the complex relationship between respiratory pathogens, their host and its environment, respiratory disease has still managed to emerge as a significant economic problem for producers. Recently, many high-health status herds have fallen victim to severe respiratory disease outbreaks. As no one etiologic agent by itself could be identified as the sole cause of the problem, it prompted the swine community to appropriately refer to this condition as the Porcine Respiratory Disease Complex (PRDC). This complex is most often due to the interaction of multifactorial etiologies. Both viral and bacterial organisms play a role, as does the environment, and various management practices employed by producers. When in the right combination, these factors can sufficiently compromise the pigs respiratory defense mechanisms, resulting in severe respiratory disease. Most often this occurs 8 to 10 weeks following placement into the finishing facilities. This is commonly referred to as the "18 to 20 week wall", and manifests itself a marked decrease in performance due to severe outbreaks of respiratory disease. In fact, when PRDC outbreaks occur in the finishing area, producers can expect elevated mortality, decreased feed efficiency, higher cull rates, increased days to market, and increased treatment costs. The Porcine Respiratory Disease Complex has been reported to have resulted in morbidity as high as 70 % with mortality averaging between 4 to 6 %.¹ According to a survey by the National Animal Health Monitoring Systems (NAHMS) on swine health and management reported that the costs of PRDC can range from $0.21/pig in feeder pigs to $1.34/pigs in a farrow to finish operation.⁴

The Porcine Respiratory Disease Complex is most often due to the interaction and synergy of both viral and bacterial pathogens. The extent of respiratory disease may be exacerbated when the appropriate environmental conditions and/or management practices are combined with various pathogens. The most common viral pathogens associated with the PRDC are porcine reproductive and respiratory syndrome (PRRS), swine influenza (SIV), pseudorabies virus (PRV), and porcine respiratory corona virus (PRCV). These viruses are usually isolated in addition to one or more bacterial pathogens. The bacterial pathogens associated with PRDC are classified as primary or secondary pathogens. The primary pathogens are capable of causing respiratory disease by themselves, where as the secondary pathogens must follow either a virus or a primary bacterial pathogen in order to cause disease. The most common primary bacterial pathogens associated with this complex include: Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, and Bordetella bronchioseptica. The secondary bacterial pathogens commonly reported include: Pasteurella multocida, Haemophilus parasuis, Streptococcus suis, Actinomyces pyogenes, Salmonella choleraesuis, and Actinobacillus suis.

In addition to the infectious causes of PRDC, various non-infectious, environmental and management factors may contribute to the development of this condition. Commonly, severe environmental stresses such as: chilling, temperature fluctuations, elevated humidity, overcrowding, frequent mixing of pigs, continuous flow production, excessive ammonia levels (> 50 ppm), and significant ascarid larval migration have been reported to predispose to development of PRDC. Environmental stressors, like the infectious agents associated with PRDC, can significantly suppress the pigs anatomic, and cellular respiratory defense mechanisms. Various management practices may also have a contributory role in the development of PRDC. Dee, 1996, reported that inadequate gilt replacement programs, large populations of pigs with subpopulations of immunity, wide ranges in weekly weaning ages, and commingling of piglets, as management practices that may help establish PRDC within a herd.² Many investigators are uncertain as to why PRDC typically expresses itself at the 18 to 20 week period. However, some speculate that it is due to the existence of subpopulations of pigs within a herd with different levels of immunity.^1,2,3,4 When pigs from multiple sites are mixed with pigs having a different immune status, those harboring subclinical disease spread it to the other pigs which may be naive to that disease, and the result is outbreaks in the naive populations. Management practices such as mixing multiple sources of pigs into large nurseries, improper gilt replacement programs, and commingling of piglets with different levels of immunity, can lead to persistence of PRDC within a herd.¹

An exact diagnosis as to the cause of PRDC within a herd is difficult to accomplish due to the multifactorial etiologies usually involved. Typically, producers will see clinical signs consisting of acute depression, anorexia, fever, nasal discharge, ocular discharge, coughing, and labored breathing. However, not all cases are the same, some may be less severe than others, and in contrast, some may be more severe, with acute death and severe depression. Therefore, in order to diagnosis PRDC it is important to gather a detailed clinical history including age of onset, morbidity and mortality estimates, response to treatment, and the most current vaccination status of the sows and pigs. In addition, cross sectional serological profiles, and necropsies are recommended as diagnostic tools to help define where in the production system the disease is occurring, and to help evaluate and assess the immune status of the pigs within these areas. Also paramount to an accurate diagnosis is collection and submission of appropriate tissue samples. Improper samples will not be able to provide an accurate evaluation of the disease processes which are occurring, nor will they allow accurate isolation and/ or culture of pathogenic organisms.

The Porcine Respiratory Disease Complex presents a substantial challenge to both veterinarians and producers. In order, to combat this problem and maintain control of it, strict management polices, and environmental monitoring will have to be consistently maintained. Implementing strategic vaccination programs directed at stabilizing and developing a uniform immune status within the sow herd to eliminate subpopulations is crucial. Consistent vaccination schedules, along with all in/ all out facilities, strict biosecurity, segregated early weaning, and the use of multisite production, should aid in the prevention and severity of PRDC.

REFERENCES

(1) Dee, S.A. The PRDC: Are Subpopulations important? Swine Health and Production. 1996; 4; 3: 147-149.

(2) Dee, S.A. Porcine Respiratory Disease Complex The "18 Week Wall". PIGS-Misset 1997; 13; 1: 13.

(3) Hillyer, G. Plight of PRDC. Hogs Today, May 1997: 13.

(4) Hoover, T. Porcine Respiratory Disease Complex. Pfizer Animal Health Technical Bulletin. October 1996.